The present invention relate to valve-regulated lead-acid (VRLA) batteries that are suitable for use in hybrid electric vehicles (HEVs) and electric vehicles (EVs).
Exhaust emissions from transport vehicles are a major cause of both greenhouse gas build-up and urban pollution. Concern over these issues has resulted in the introduction of new anti-pollution legislation that significantly restricts exhaust emissions from internal combustion engines. Some countries have been more severe in their approach and have legislated that a certain number of vehicles sold must have either low or zero emissions. Such vehicles include electric vehicles (EVs) and hybrid electric vehicles (HEVs). The success of this initiative hinges on the development of vehicles that have both appropriate performance and lifetime cost characteristics.
HEV battery packs are subjected to multiple charge-discharge cycles below a full state-of-charge (SoC). Such duty can cause a localized, irreversible build-up of lead sulphate. This impairs battery performance. Similar buildups, along with associated high temperatures and uneven temperature gradients can also occur within EV batteries that are subjected to rapid recharge and discharge conditions.
The specification of U.S. Pat. No. 4,760,001 discloses a battery comprising negative plates made from expanded lead-coated copper having tabs formed by a copper strip extending across the plate. In one form of the battery, the copper strip extends beyond exposed edges of the negative plate to form lugs or tabs on opposite sides of the plate. This leads to sub-optimal location of the tabs with respect to drainage of current and heat. Furthermore, lead-coated expanded copper plate are considerably more expensive to make than expanded lead plates. In addition, such batteries would not be suited to HEV or EV use because of their high cost and additional weight.
The specification of U.S. Pat. No. 4,983,475 discloses a battery design in which each plate has dual tabs on opposed sides and each tab is connected to a corresponding negative or positive busbar. Each of the busbars are in turn connected by diagonally disposed straps. The purpose of the dual tabs and straps is to improve the electrical characteristics of the battery. However, the batteries described in the specification would not be suitable for HEV and EV use because they are only 2 volt batteries and the straps add unnecessary weight. Furthermore, the straps absorb valuable space.
The specification of U.S. Pat. No. 4,603,093 discloses battery cells having two or more tabs per plate. The purpose of the multiple tabs is to improve energy density and power density. This design permits the use of longer shallower plates than previously contemplated. However, the multiple tabs are located on one side of the plate.
The specification of WO 99/40,638 describes cells having plates of the opposite geometry as that described in the specification of U.S. Pat. No. 4,603,093. In other words, the plates are narrow and deep. In order to improve the availability of current from cells containing plates of this design, tabs are placed on opposite sides of the plate and current from one end is transferred to the other by means of a lead-plated copper strap. This improves current availability because copper is a better conductor than lead. Although this design includes tabs on opposed sides of the plate, it does not contemplate terminals on opposed sides of the battery. Consequently, current still has to be transferred from one side of the plate to the other in order to connect with the relevant terminal. Furthermore, the strap adds to the weight of the battery.
In one aspect, the present invention provides a valve regulated lead acid (VRLA) cell comprising a positive and negative plate separated by a separator and held together under pressure. Preferably, the pressure applied to the cell lies in the range from 20 to 100 kPa. The separator supports therein an electrolyte. Each plate has a first single or plurality of tabs on a first side of the plate, and a second single or plurality of tabs on a second side of the plate. Each tab is connected to a busbar to form positive and negative busbars on each of the first and second sides of the plate.
The cell may be a spirally-wound cell, or a prismatic cell. The spirally-wound cells may be either 2V cells, or manufactured to produce monoblocs with a total voltage of 4 and higher. Spirally-wound cells have current takeoffs at both the top and bottom of the both negative and positive plated (hitherto referred to as spirally-wound batteries with bi-directions current takeoffs). The prismatic cell preferably includes a plurality of such positive and negative plates separated by separators. A plurality of cells may be connected in series.
In another aspect, the invention provides a VRLA battery comprising a plurality of cells joined in series, wherein each cell includes one or more positive and negative plates separated by one or more separators and held together under pressure. Preferably, the pressure applied to the cell lies in the range from 20 to 100 kPa. The separator supports therein an electrolyte. Each plate has a first single or plurality of tabs on a first side of the plate, and a second single or plurality of tabs on a second side of the plate. Each tab is connected to a busbar to form positive and negative busbars on each of the first and second sides of the plate. Each cell may be connected to a neighboring cell by welded joints between alternate positive and negative busbars. These welds are preferably, but not exclusively, through the cell-case wall or over the top of the cell wall. Each cell may be independently sealed airtight. Alternatively, all the cells in the battery may have a common head-space. A plurality of batteries may be connected in series.
The separator used in the invention can be made of absorptive-glass micro-fiber, or can be compatible with the use of gelled-electrolyte. Alternatively, any separator material that can withstand reasonable levels of compression (for example, pressure greater than 20 kPa) is suitable.
In another aspect, the invention provides an electric or electric hybrid vehicle (eg., EV or HEV) that includes one or more such cells or batteries.
The invention provides several advantages. VRLA cells and batteries of the invention are light-weight and low cost. Such cells and batteries have the capacity to deliver substantial current flows while in a partial-state-of-charge (PSoC) condition over a large number of cycles. Also, under high charge and discharge conditions, cells and batteries according to the present invention maintain a much lower and almost isothermal internal battery temperature, compared to that experienced in prior art designs. The dual-tab design does not develop significant temperature gradients during either HEV or PSoC/fast-charge EV duty and does not suffer from preferential sulphation. All these features provide distinct advantages for vehicles applications.